The invention relates to a method and an apparatus for fabricating a semiconductor device, and more particularly to such a method including a step of patterning multi-layered metal wirings composed of aluminum, through inducive coupling plasma, and also to such an apparatus for generating inducive coupling plasma to thereby pattern multi-layered metal wirings.
A wiring layer is required to be patterned in a size smaller and smaller with enhancement in integration of LSI. In order to transfer a small-sized mask pattern to a layer such as an electrically conductive layer and an insulating layer, such a layer is usually etched by anisotropic dry etching which makes use of plasma, such as reactive ion etching (RIE) and electron cyclotron resonance (ECR) plasma etching.
On the other hand, as a semiconductor device has been fabricated in a size smaller and smaller, a gate insulating film in an insulating gate type field effect transistor (FET) is made thinner and thinner. For instance, a latest gate insulating film is designed to have a thickness of 10 nm or smaller. Such a thin gate insulating film is likely to be damaged by just small electric stress.
An electrically conductive layer is usually composed of aluminum or alloy thereof. There are known a couple of anisotropic dry etching methods in which plasma is generated, as methods for dry-etching such an electrically conductive layer.
For instance, in one of such dry etching methods, an electrically conductive layer composed of aluminum or alloy thereof, formed on a semiconductor substrate, is etched with a patterned resist film being used as a mask through plasma generated from a mixture gas of and Cl2 gas. In plasma, the BCl3 gas exists as BCl2+, and the Cl2 gas produces Cl radicals. These Cl species make chemical reaction with aluminum or alloy thereof of which the electrically conductive layer is composed, to thereby generate volatile AlCl3 having high vapor pressure. The thus generated volatile AlCl3 is evaporated, and resultingly, the electrically conductive layer composed of aluminum or alloy thereof is etched.
In a plasma etching process, electrons as well as the above-mentioned Cl ion species are incident to a substrate. If there is a difference between incident positive and negative electric charges, electric charges are accumulated in an electrically conductive layer which is formed on a gate insulating film and is electrically insulated from the substrate. As a result, there is generated a difference in a voltage between the electrically conductive layer and the substrate. Such a difference in a voltage allows tunnel current to pass through the gate insulating film, resulting in that dielectric characteristics of the gate insulating film is varied, and hence, the gate insulating film might reach dielectric breakdown.
As mentioned above, a plasma etching process in which a gate electrode or an electrically conductive layer formed on a gate insulating film, and electrically conductive layers electrically connected to the gate electrode (hereinafter, a gate electrode and such electrically conductive layers are referred to as xe2x80x9cgate wiringsxe2x80x9d) are charged up, that is, electric charges are accumulated in the gate wirings, might cause the gate insulating film to be damaged. A plasma etching process is carried out, for instance, when a gate wiring layer is patterned, when a contact hole is formed reaching a gate wiring layer, when a contact hole reaching a gate insulating layer is cleaned by sputter etching, and when plasma-enhance chemical vapor deposition is carried out to a surface of a substrate at which a gate wiring layer partially appears.
In addition, if plasma formed non-uniformly above a semiconductor substrate, there would be generated a difference between an ion current and an electron current both to be introduced into the semiconductor substrate. This difference might allow a tunnel current to pass through a gate insulating film.
As a semiconductor device is integrated highly and highly, an antenna ratio defined as a ratio of an area of a gate insulating film to an area of an electrically conductive layer composed of aluminum or alloy thereof is significantly increased. When an electrically conductive film having a high antenna ratio is to be etched by a plasma etching process, small non-uniformity in plasma might allow much tunnel current to pass through a gate insulating film.
It has been reported that even if there is generated plasma having a profile of electric charges which is uniform to a flat surface, there would occur charging damage called electron shading damage inherent to high-density plasma etching, in a plasma etching step in which a resist mask including an aperture having a high aspect ratio, that is, a narrow space.
The above-mentioned electron shading damage is caused by both charge imbalance between ion flux and electron flux at a bottom of a space formed between wirings, and micro-loading which is one of characteristics of dry etching.
Hereinbelow is explained the electron shading damage with reference to FIG. 1.
With reference to FIG. 1, in ion sheath formed in an electrode to which RF voltage is applied when plasma is discharged, ion flux 101 is anisotropically incident to spaces formed between adjacent metal wirings 105, whereas electron flux 102 is isotropically incident to the spaces in the same manner as electron flux being incident in plasma bulk.
Anisotropy of the electron flux 102 causes a majority of electrons 104 is incident onto sidewalls of an insulating mask such as resist masks 103. As a result, the electrons 104 are much accumulated on the sidewalls of the resist masks 103 to thereby generate a negative voltage on the resist masks 103. As a space between the resist masks 103 is small, that is, as an aspect ratio is high, negative voltages generated on the sidewalls of the resist masks 103 overlap each other, resulting in that the negative voltages generated around the resist masks 103 are further increased. Accordingly, an amount of the electron flux 102 reaching bottoms 106 of holes formed between the adjacent resist masks 103 is significantly reduced. Thus, there is generated imbalance between the electron flux 102 and the ion flux 101 at the bottoms 106 of the holes having a high aspect ratio.
Dry etching has many characteristics, one of which is micro-loading effect. Herein, the micro-loading effect is a phenomenon in which an etching rate varies in dependence on an aspect ratio. In general, an etching rate lowers as an aspect ratio increases. Hence, though the metal wirings 105 have been already etched in portions having a relatively low aspect ratio, the bottoms 106 of the holes having a relatively high aspect ratio have not been etched yet. In a period of time after the portions having a relatively low aspect ratio have been etched for removal until the bottoms 106 have been completely etched (hereinafter, such a period of time is referred to as xe2x80x9cinjection timexe2x80x9d), positive electric charges are accumulated on the bottoms 106 due to charge imbalance.
As a result, a gate electrode 108 has a positive voltage relative to a silicon substrate 109. Then, electrons 111 are injected into the gate electrode 108 from the silicon substrate 109 through a gate oxide film 110 in order to dissolve the charge imbalance. A current caused by the injected electrons 111 is in proportion to both a gate voltage and the above-mentioned injection time.
If a current caused by the injected electrons 111 flow excessively through the gate oxide film 110, the gate oxide film 110 would be degraded and/or damaged.
The explanation mentioned above is the reason why the electron shading damage occurs.
Many attempts have been made to uniformize plasma by generating plasma by virtue of pulse modulation. For instance, Japanese Unexamined Patent Publications Nos. 6-267900 and 8-181125 have suggested methods of etching a wiring layer. In the methods, an ECR plasma source applies RF bias voltage having a frequency of 600 kHz or smaller to a gas with a pulse cycle being 100 xcexcsec or smaller and pulse-off time being in the range of 10 to 100 xcexcsec. As a result, positive and negative ions are effectively produced, and thus, electric charges are not accumulated on sidewalls of a resist mask.
In particular, Japanese Unexamined Patent Publication No. 6-267900 teaches that it would be possible to prevent micro-loading which occurs when a contact hole having a high aspect ratio is to be formed by etching, by generating plasma which is modulated with pulses having a frequency of 50 kHz.
Japanese Unexamined Patent Publication No. 61-13625 has suggested a plasma-etching apparatus comprising first means for generating plasma from process gas introduced into a reaction chamber, second means for accelerating ions in plasma to thereby radiate the thus accelerated ions to an object, third means for modulating a discharge voltage, and fourth means for modulating an applied voltage. The fourth means AM- or FM-modulates an applied voltage to thereby control a profile of electron temperature, a composition ratio of reaction species, and an ion energy profile.
The above-mentioned conventional plasma etching apparatuses, in particular, ECR plasma etching apparatus make use of expensive magnets. In addition, it is reported that those magnets would degrade electric characteristics of a sample. For this reason, an inducive coupling plasma (ICP) etching apparatus is presently suggested in place of the above-mentioned conventional plasma etching apparatuses. Though the ICP etching apparatus generates plasma having a smaller density than a density of plasma generated in ECR plasma etching apparatus, the ICP etching apparatus has a simpler structure than that of ECR plasma etching apparatus, because the ICP etching apparatus includes no magnets. In addition, the ICP etching apparatus has advantages of less electric damage to a gate insulating film, higher mechanical reliability, higher maintenance ability and lower fabrication and running costs relative to the other etching apparatuses.
Japanese Unexamined Patent Publication No. 9-92645 has suggested a method of fabricating a semiconductor device, comprising the steps of transferring a semiconductor wafer into a plasma etching chamber, and applying plasma to a semiconductor wafer. The semiconductor wafer includes a gate insulating film having a dielectric breakdown voltage of X volts and a thickness of 10 nm or smaller, an electrically conductive layer formed on the gate insulating film and having an antenna structure having an antenna ratio of 500 or greater, and a patterned insulator formed on the electrically conductive layer and including apertures each having an aspect ratio greater than 1. In plasma, electron temperature Te (eV) is kept equal to or smaller than X (Texe2x89xa6X).
In the suggested method, RF voltage having a frequency of 13.56 MHz is applied to process gas to thereby plasma in ICP etching apparatus, and RF power having a frequency of 66.7 kHz is applied to a substrate to thereby control a substrate voltage. RF signals having waveforms analogous to RF output waveforms are detected from an RF bias voltage source, and the thus detected RF signals are transmitted to a pulse generator. The pulse generator generates pulses having desired pulse-on time which is synchronized to a desired phase in a cycle period equal to a cycle period of the received RF signals. The pulses generated by the pulse generator are transmitted to a power source to modulate or turn on or off RF power having a frequency of 13.56 MHz, in accordance with the pulses. Specifically, pulse-on time is set equal to 5 xcexcsec, a pulse-off time is set equal to 10 xcexcsec, and a phase angle is set equal to 240 degrees. Thus, it would be possible to keep an electron temperature below a dielectric breakdown voltage of a gate insulating film.
According to the Publication, as a pulse-off time is set longer, a period of time in which an electron temperature is reduced becomes longer, and an electron temperature at the end of the period is further reduced. The Publication sets forth that remarkable reduction in damage to a gate insulating layer is dependent on the above-mentioned reduction in an electron temperature.
It is an object of the present invention to provide a method for fabricating a semiconductor device, including a step of patterning a wiring layer through inducive coupling plasma, which method makes it possible to prevent occurrence of electron shading damage to a gate insulating film and etch a wiring layer with high accuracy-and high reliability.
It is also an object of the present invention to provide an apparatus for fabricating a semiconductor device, which apparatus generates inducive coupling plasma to thereby etch a wiring layer without occurrence of electron shading damage to a gate insulating film.
In one aspect of the present invention, there is provided a method of fabricating a semiconductor device, including the steps of (a) generating plasma in the following conditions: (a1) an RF bias voltage has a frequency equal to or greater than 1 MHz, (a2) an RF source voltage has a frequency equal to or greater than 1 MHz, (a3) the RF source voltage is modulated by pulses in a cycle equal to or greater than 100 xcexcsec, and (a4) pulse-on time is equal to or greater than 50 xcexcsec, and (b) patterning multi-layered metal wirings by etching through the plasma.
It is preferable that the pulses have a rectangular waveform.
It is preferable that the cycle is equal to or smaller than 500 xcexcsec, and the pulse-on time is equal to or smaller than 450 xcexcsec.
It is preferable that the multi-layered metal wirings are composed of aluminum or aluminum alloy, in which case, the multi-layered metal wirings preferably make electrical contact with an n-channel MOSFET. It is also preferable that the n-channel MOSET includes a gate insulating film having a thickness equal to or smaller than 6 nm.
It is preferable that the multi-layered metal wirings have an antenna ratio in the range of 1,000 to 100,000 both inclusive, and wirings in the multilayered metal wirings are spaced away from adjacent ones by 0.3 xcexcm or greater.
It is preferable that the multi-layered metal wirings have an antenna ratio in the range of 1,000 to 40,000 both inclusive.
It is preferable that pulse-off time is equal to or smaller than 100 xcexcsec, and that the pulse-off time is equal to or greater than 20 xcexcsec, preferably than 30 xcexcsec.
In another aspect of the present invention, there is provided an apparatus for fabricating a semiconductor device, including (a) a hermetically sealed chamber, (b) a gas introducer for introducing gas into the chamber, (c) a gas exhauster for exhausting gas from the chamber, (d) an RF source voltage supplier applying an RF source voltage having a frequency equal to or greater than 1 MHz to the chamber to thereby generate inducive coupling plasma from the gas, (e) an RF bias voltage source applying an RF bias voltage to a substrate put in the chamber, the RF bias voltage having a frequency equal to or greater than 1 MHz, and (f) a pulse generator which transmits pulses to the RF source voltage supplier to thereby modulate the RF source voltage supplier in a cycle equal to or greater than 100 xcexcsec with pulse-on time being kept equal to or greater than 50 xcexcsec.
It is preferable that the pulse generator generates pulses each having a rectangular waveform.
It is preferable that the pulse generator transmits pulses to the RF source voltage supplier to thereby modulate the RF source voltage supplier in a cycle equal to or smaller than 500 xcexcsec with the pulse-on time being kept equal to or smaller than 450 xcexcsec.
It is preferable that the semiconductor device includes multi-layered metal wirings composed of aluminum or aluminum alloy, in which case, it is also preferable that the semiconductor device further includes an n-channel MOSFET with which the multi-layered metal wirings make electrical contact. It is preferable that the n-channel MOSET includes a gate insulating film having a thickness equal to or smaller than 6 nm.
It is preferable that the pulse generator transmits pulses to the RF source voltage supplier to thereby modulate the RF source voltage with pulse-off time being equal to or smaller than 100 xcexcsec, but equal to or greater than 20 xcexcsec, preferably than 30 xcexcsec.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In accordance with the present invention, conditions for generating plasma when multi-layered metal wirings are to be patterned are determined as follows.
A frequency of the RF bias voltage: 1 MHz or greater
A frequency of the RF source voltage: 1 MHz or greater
A cycle in which the RF source voltage is modulated: 100 xcexcsec or greater
Pulse-on time: 50 xcexcsec or greater
By determining the conditions as mentioned above, it would be possible to reduce charging damage to a gate insulating film, even if wirings are further spaced away from adjacent ones and/or an antenna ratio of the multi-layered metal wirings is further increased
It should be noted that conditions for generating plasma are not to be limited to the above-mentioned ones, but may be varied within the scope of the present invention.
In addition, the present invention is preferably applied to multi-layered metal wirings composed of aluminum or alloy thereof, but may be applied to a layer composed of other materials such as polysilicon or silicon dioxide.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.